In vivo satellite cell activation via Myf5 and MyoD in regenerating mouse skeletal muscle

In vivo imaging of tumor-propagating cells, regional tumor heterogeneity, and dynamic cell movements in embryonal rhabdomyosarcoma

Embryonal rhabdomyosarcoma (ERMS) is an aggressive pediatric sarcoma of muscle. Here, we show that ERMS-propagating potential is confined to myf5+ cells and can be visualized in live, fluorescent transgenic zebrafish. During early tumor growth, myf5+ ERMS cells reside adjacent normal muscle fibers. By late-stage ERMS, myf5+ cells are reorganized into distinct regions separated from differentiated tumor cells. Time-lapse imaging of late-stage ERMS revealed that myf5+ cells populate newly formed tumor only after seeding by highly migratory myogenin+ ERMS cells. Moreover, myogenin+ ERMS cells can enter the vasculature, whereas myf5+ ERMS-propagating cells do not. Our data suggest that non-tumor-propagating cells likely have important supportive roles in cancer progression and facilitate metastasis.

Single muscle-fiber isolation and culture for cellular, molecular, pharmacological, and evolutionary studies

The technique of single muscle-fiber cultures has already proven valuable in extending knowledge of myogenesis, stem cell heterogeneity, the stem cell niche in skeletal muscle, and satellite cell activation. This report reviews the background of the model and applications, and details the procedures of muscle dissection, fiber digestion and isolation, cleaning the fiber preparation, plating fibers, and extensions of the technique for studying activation from stable quiescence of satellite cells, mRNA expression by in situ hybridization and regulation of satellite cell activation in zebrafish muscle by nitric oxide, hepatocyte growth factor.

Testosterone improves the regeneration of old and young mouse skeletal muscle

Aging is associated with loss of muscle mass and strength, reduced satellite cell number, and lower regenerative potential. Testosterone increases muscle mass, strength, and satellite cell number in humans; however, the effects of testosterone on the regenerative potential of skeletal muscle are unclear. Here, we investigated the effect of testosterone on the skeletal muscle regeneration of young (2-month-old) and aged (24-month-old) male mice. We show that testosterone increases the number of proliferating satellite cells in regenerating "tibialis anterior" muscle of young and aged castrated mice 2 and 4 days postinjury. Testosterone supplementation increases the number and the cross-sectional area of regenerating fibers in both classes of age 4 days postinjury. Testosterone increases satellite cell activation and proliferation and the regeneration of both young and aged mouse muscle. These data suggest prospective application of androgens to improve the regenerating potential of the aged human skeletal muscle.

Sex, fiber-type, and age dependent in vitro proliferation of mouse muscle satellite cells

During postnatal growth and after muscle injury, satellite cells proliferate and differentiate into myotubes to form and repair musculature. Comparison of studies on satellite cell proliferation and differentiation characteristics is confounded by the heterogeneity of the experimental conditions used. To examine the influence of sex, age, and fiber-type origin on in vitro properties of satellite cells derived from postnatal muscles, fast extensor digitorum longus (EDL) and slow soleus (SOL) muscles were extracted from male and female mice of 1 week to 3 months of age. Myoblast proliferation and myogenic regulatory factor (MRF) expression was measured from cultures of freshly isolated satellite cells. Higher proliferation rate and elevated Myod1 expression was found in male EDL and SOL derived cells compared with females at age of 40, 60, and 120 days, whereas inverse tendency for cell proliferation was apparent in EDL of juvenile (7-day-old) pups. Myogenin and Mrf4 transcripts were generally elevated in males of 40 and 60 days of age and in female EDL of juveniles. However, these differentiation markers did not significantly correlate with proliferation rate at all ages. Pax7, whose overexpression can block myogenesis, was up-regulated especially in 40-day-old females where MRF expression was low. These results indicate that gender, postnatal age, and muscle fiber origin affect proliferation and muscle transcription factor expression in vitro. The results also support the view that satellite cells originating from slow and fast muscles are intrinsically different and warrant further studies on the effect of cell origin for therapeutic approaches.

Caveolins in rhabdomyosarcoma

Caveolins are scaffolding proteins that play a pivotal role in numerous processes, including caveolae biogenesis, vesicular transport, cholesterol homeostasis and regulation of signal transduction. There are three different isoforms (Cav-1, -2 and -3) that form homo- and hetero-aggregates at the plasma membrane and modulate the activity of a number of intracellular binding proteins. Cav-1 and Cav-3, in particular, are respectively expressed in the reserve elements (e.g. satellite cells) and in mature myofibres of skeletal muscle and their expression interplay characterizes the switch from muscle precursors to differentiated elements. Recent findings have shown that caveolins are also expressed in rhabdomyosarcoma, a group of heterogeneous childhood soft-tissue sarcomas in which the cancer cells seem to derive from progenitors that resemble myogenic cells. In this review, we will focus on the role of caveolins in rhabdomyosarcomas and on their potential use as markers of the degree of differentiation in these paediatric tumours. Given that the function of Cav-1 as tumour conditional gene in cancer has been well-established, we will also discuss the relationship between Cav-1 and the progression of rhabdomyosarcoma.

In ovo feeding of IGF-1 to ducks influences neonatal skeletal muscle hypertrophy and muscle mass growth upon satellite cell activation

To investigate reasons for the muscle increase observed when eggs are treated by IGF-1 and whether or not satellite cell activation is specific to different types of myofibers, duck eggs were administrated with IGF-1. After injection, during the neonatal stages, the duck breast muscle and leg muscle were isolated for analysis. The muscle weight, muscle fiber diameter (MFD), cross-sectional area (CSA), the number of myofibers per unit area (MFN) and frequency of satellite cell activation and mitosis at the embryo stage of 27 days (27E) and the postnatal stage of 2 days after hatching (P2D) were determined. In addition, expression of two important myogenic transcription factors MyoD and Myf5 were detected and compared in the two types of muscle tissues. Results indicated that IGF-1 administration increased the duck body weight, MFD, CSA, MFN, and quantity of activated satellite cells and mitotic nuclei in the two types of muscle tissues. The MyoD and Myf5 expressed at a higher level in IGF-1-treated muscle. IGF-1 stimulated muscle weight growth more in the leg muscle than in the breast muscle. These results indicate that in ovo feeding of IGF-1 can stimulate duck growth and, especially, lead to increased muscle hypertrophy. These increases appear to be mainly dependent on the activation of satellite cells, some of which proliferate and fuse to the myofiber, enabling increased muscle mass. IGF-1 can indirectly affect satellite cells by regulating the expression of two important myogenic transcription factors, MyoD and Myf5, which help activate satellite cells.

Unloading stress disturbs muscle regeneration through perturbed recruitment and function of macrophages

Skeletal muscle is one of the most sensitive tissues to mechanical loading, and unloading inhibits the regeneration potential of skeletal muscle after injury. This study was designed to elucidate the specific effects of unloading stress on the function of immunocytes during muscle regeneration after injury. We examined immunocyte infiltration and muscle regeneration in cardiotoxin (CTX)-injected soleus muscles of tail-suspended (TS) mice. In CTX-injected TS mice, the cross-sectional area of regenerating myofibers was smaller than that of weight-bearing (WB) mice, indicating that unloading delays muscle regeneration following CTX-induced skeletal muscle damage. Delayed infiltration of macrophages into the injured skeletal muscle was observed in CTX-injected TS mice. Neutrophils and macrophages in CTX-injected TS muscle were presented over a longer period at the injury sites compared with those in CTX-injected WB muscle. Disturbance of activation and differentiation of satellite cells was also observed in CTX-injected TS mice. Further analysis showed that the macrophages in soleus muscles were mainly Ly-6C-positive proinflammatory macrophages, with high expression of tumor necrosis factor-α and interleukin-1β, indicating that unloading causes preferential accumulation and persistence of proinflammatory macrophages in the injured muscle. The phagocytic and myotube formation properties of macrophages from CTX-injected TS skeletal muscle were suppressed compared with those from CTX-injected WB skeletal muscle. We concluded that the disturbed muscle regeneration under unloading is due to impaired macrophage function, inhibition of satellite cell activation, and their cooperation.

Myf5 haploinsufficiency reveals distinct cell fate potentials for adult skeletal muscle stem cells

Skeletal muscle stem cell fate in adult mice is regulated by crucial transcription factors, including the determination genes Myf5 and Myod. The precise role of Myf5 in regulating quiescent muscle stem cells has remained elusive. Here we show that most, but not all, quiescent satellite cells express Myf5 protein, but at varying levels, and that resident Myf5 heterozygous muscle stem cells are more primed for myogenic commitment compared with wild-type satellite cells. Paradoxically however, heterotypic transplantation of Myf5 heterozygous cells into regenerating muscles results in higher self-renewal capacity compared with wild-type stem cells, whereas myofibre regenerative capacity is not altered. By contrast, Pax7 haploinsufficiency does not show major modifications by transcriptome analysis. These observations provide a mechanism linking Myf5 levels to muscle stem cell heterogeneity and fate by exposing two distinct and opposing phenotypes associated with Myf5 haploinsufficiency. These findings have important implications for how stem cell fates can be modulated by crucial transcription factors while generating a pool of responsive heterogeneous cells.

Injection of duck recombinant follistatin fusion protein into duck muscle tissues stimulates satellite cell proliferation and muscle fiber hypertrophy

Follistatin (FST) can inhibit the expression of myostatin, which is a predominant inhibitor of muscle development. The potential application of myostatin-based technology has been prompted in different ways in agriculture. We previously constructed an expression vector of duck FST and isolated the FST fusion protein. After the protein was purified and refolded, it was added to the medium of duck myoblasts cultured in vitro. The results show that the 3-(4,5-dimethylthiazol-2-yl)-2, 5-diphenyltetrazolium bromide value of the myoblasts in the duck FST treatment group is higher than that in the control group, which indicates that the duck FST fusion protein exhibits the biological activities that can accelerate myoblast proliferation. To further investigate the roles of duck FST on muscle development, we injected the protein into the duck muscle tissues in vivo. The results show that both the duck muscle fiber cross-sectional area and the satellite cell activation frequency are influenced more in the FST treatment group than they are in the control group. In addition to these phenomena, expression of MyoD and Myf5 were increased, and the expression of myostatin was decreased. Together, these results suggest the potential for using duck FST fusion protein to inhibit myostatin activity and subsequently to enhance muscle growth in vivo. The mechanism by which FST regulates muscle development in the duck is similar to that in mammals and fishes.

Skeletal muscle regeneration is delayed by reduction in Xin expression: consequence of impaired satellite cell activation?

Xin is a striated muscle-specific actin-binding protein whose mRNA expression has been observed in damaged skeletal muscle. Here we demonstrate increased Xin protein expression early postinjury (≤12 h) and localization primarily to the periphery of damaged myofibers. At 1 day postinjury, Xin is colocalized with MyoD, confirming expression in activated satellite cells (SCs). By 5 days postinjury, Xin is evident in newly regenerated myofibers, with a return to preinjury levels by 14 days of regeneration. To determine whether the increased Xin expression is functionally relevant, tibialis anterior muscles of wild-type mice were infected with Xin-short hairpin RNA (shRNA) adenovirus, whereas the contralateral tibialis anterior received control adenovirus (Control). Four days postinfection, muscles were harvested or injured with cardiotoxin and collected at 3, 5, or 14 days thereafter. When compared with Control, Xin-shRNA infection attenuated muscle regeneration as demonstrated by Myh3 expression and fiber areas. Given the colocalization of Xin and MyoD, we isolated single myofibers from infected muscles to investigate the effect of silencing Xin on SC function. Relative to Control, SC activation, but not proliferation, was significantly impaired in Xin-shRNA-infected muscles. To determine whether Xin affects the G0-G1 transition, cell cycle reentry was assessed on infected C2C12 myoblasts using a methylcellulose assay. No difference in reentry was noted between groups, suggesting that Xin contributes to SC activation by means other than affecting G0-G1 transition. Together these data demonstrate a critical role for Xin in SC activation and reduction in Xin expression results in attenuated skeletal muscle repair.

Myogenic progenitors contribute to open but not closed fracture repair

BACKGROUND

Bone repair is dependent on the presence of osteocompetent progenitors that are able to differentiate and generate new bone. Muscle is found in close association with orthopaedic injury, however its capacity to make a cellular contribution to bone repair remains ambiguous. We hypothesized that myogenic cells of the MyoD-lineage are able to contribute to bone repair.

METHODS

We employed a MyoD-Cre+:Z/AP+ conditional reporter mouse in which all cells of the MyoD-lineage are permanently labeled with a human alkaline phosphatase (hAP) reporter. We tracked the contribution of MyoD-lineage cells in mouse models of tibial bone healing.

RESULTS

In the absence of musculoskeletal trauma, MyoD-expressing cells are limited to skeletal muscle and the presence of reporter-positive cells in non-muscle tissues is negligible. In a closed tibial fracture model, there was no significant contribution of hAP+ cells to the healing callus. In contrast, open tibial fractures featuring periosteal stripping and muscle fenestration had up to 50% of hAP+ cells detected in the open fracture callus. At early stages of repair, many hAP+ cells exhibited a chondrocyte morphology, with lesser numbers of osteoblast-like hAP+ cells present at the later stages. Serial sections stained for hAP and type II and type I collagen showed that MyoD-lineage cells were surrounded by cartilaginous or bony matrix, suggestive of a functional role in the repair process. To exclude the prospect that osteoprogenitors spontaneously express MyoD during bone repair, we created a metaphyseal drill hole defect in the tibia. No hAP+ staining was observed in this model suggesting that the expression of MyoD is not a normal event for endogenous osteoprogenitors.

CONCLUSIONS

These data document for the first time that muscle cells can play a significant secondary role in bone repair and this knowledge may lead to important translational applications in orthopaedic surgery. Please see related article: http://www.biomedcentral.com/1741-7015/9/136.

Leucine limitation regulates myf5 and myoD expression and inhibits myoblast differentiation

Satellite cells are the major pool of muscle stem cells after birth; they represent an important component required to maintain muscle mass and functionality during life. The molecular mechanisms involved in myogenic differentiation are relatively well-known. However, the role of extracellular stimulus in the control of differentiation remains largely unresolved. Notably little is known about the impact of nutrients on this process. Here we have studied the role of leucine, an essential amino acid, in the control of myogenic differentiation. Leucine is a well-known regulator of muscle protein synthesis. It acts not only as a substrate for translation but also as a regulator of gene expression and signaling pathways such as those involving mTOR and GCN2. In this study we demonstrated that the lack of leucine abolishes the differentiation of both C2C12 myoblasts and primary satellite cells. This effect is associated with a modification of the pattern of expression of the myogenic regulatory factors (MRF) myf5 and myoD. We report an up-regulation of myf5 mRNA and a decrease of myoD protein level during leucine starvation. This study demonstrates the importance of a nutrient, leucine, in the control of the myogenic differentiation program.

Hesperedin promotes MyoD-induced myogenic differentiation in vitro and in vivo

BACKGROUND AND PURPOSE

The bioflavonoid, hesperedin, promotes osteoblast differentiation in human mesenchymal stem cells, indicating an anabolic effect of hesperedin on bone metabolism. Murine bone marrow mesenchymal stem cells undergo myogenic differentiation as well as osteogenic differentiation. We therefore explored whether hesperedin modulates muscle cell differentiation.

EXPERIMENTAL APPROACH

Myoblast C2C12 cells were differentiated into muscle cells in the presence or absence of hesperedin. The effects of hesperedin on myogenic differentiation were determined by analysing specific muscle markers in vitro using reporter gene assays, immunoblotting, RT-PCR and DNA pull-down assays. In vivo, the effects of hesperedin were assessed using the freeze injury-induced muscle regeneration model in mice and daily injections of hesperedin for 6 days.

KEY RESULTS

Hesperedin promoted myogenic differentiation, in a dose-dependent manner, by increasing myogenin gene expression. MyoD-induced myogenin gene transcription was enhanced by hesperedin, as this bioflavonoid augmented the nuclear localization and myogenin promoter-binding of MyoD. In addition, hesperedin increased myogenin and muscle creatine kinase gene expression during myogenic differentiation from C3H10T1/2 mesenchymal stem cells in a MyoD-dependent manner and accelerated in vivo muscle regeneration induced by muscle injury.

CONCLUSIONS AND IMPLICATIONS

Our results demonstrate that hesperedin promoted myogenic differentiation in vitro and in vivo through activation of MyoD-mediated myogenin expression, suggesting a beneficial role in promoting muscle regeneration, following injury.

Slowing down differentiation of engrafted human myoblasts into immunodeficient mice correlates with increased proliferation and migration

We have used a model of xenotransplantation in which human myoblasts were transplanted intramuscularly into immunodeficient Rag2(-/-)γC(-/-) mice, in order to investigate the kinetics of proliferation and differentiation of the transplanted cells. After injection, most of the human myoblasts had already differentiated by day 5. This differentiation correlated with reduction in proliferation and limited migration of the donor cells within the regenerating muscle. These results suggest that the precocious differentiation, already detected at 3 days postinjection, is a limiting factor for both the migration from the injection site and the participation of the donor cells to muscle regeneration. When we stimulated in vivo proliferation of human myoblasts, transplanting them in a serum-containing medium, we observed 5 days post-transplantation a delay of myogenic differentiation and an increase in cell numbers, which colonized a much larger area within the recipient's muscle. Importantly, these myoblasts maintained their ability to differentiate, since we found higher numbers of myofibers seen 1 month postengraftment, as compared to controls. Conceptually, these data suggest that in experimental myoblast transplantation, any intervention upon the donor cells and/or the recipient's microenvironment aimed at enhancing proliferation and migration should be done before differentiation of the implanted cells, e.g., day 3 postengraftment.

Teashirt-3, a novel regulator of muscle differentiation, associates with BRG1-associated factor 57 (BAF57) to inhibit myogenin gene expression

In adult muscles and under normal physiological conditions, satellite cells are found in a quiescent state but can be induced to enter the cell cycle by signals resulting from exercise, injury-induced muscle regeneration, or specific disease states. Once activated, satellite cells proliferate, self-renew, and differentiate to form myofibers. In the present study, we found that the zinc finger-containing factor Teashirt-3 (TSHZ3) was expressed in quiescent satellite cells of adult mouse skeletal muscles. We showed that following treatment with cardiotoxin TSHZ3 was strongly expressed in satellite cells of regenerating muscles. Moreover, immunohistochemical analysis indicated that TSHZ3 was expressed in both quiescent and activated satellite cells on intact myofibers in culture. TSHZ3 expression was maintained in myoblasts but disappeared with myotube formation. In C2C12 myoblasts, we showed that overexpression of Tshz3 impaired myogenic differentiation and promoted the down-regulation of myogenin (Myog) and up-regulation of paired-box factor 7 (Pax7). Moreover, knockdown experiments revealed a selective effect of Tshz3 on Myog regulation, and transcriptional reporter experiments indicated that TSHZ3 repressed Myog promoter. We identified the BRG1-associated factor 57 (BAF57), a subunit of the SWI/SNF complex, as a partner of TSHZ3. We showed that TSHZ3 cooperated with BAF57 to repress MYOD-dependent Myog expression. These results suggest a novel mechanism for transcriptional repression by TSHZ3 in which TSHZ3 and BAF57 cooperate to modulate MyoD activity on the Myog promoter to regulate skeletal muscle differentiation.

Satellite cell activity in muscle regeneration after contusion in rats

1. The role of satellite cells in muscle growth during development is well documented, but the involvement of these cells in muscle repair after contusion is less well known. In the present study, we investigated the time-course of satellite cell activity (from 3h to 7days) after contusion of rat gastrocnemius muscle using specific molecular markers for immunofluorescence and real-time polymerase chain reaction (PCR). 2. Inflammation of the injured muscle occurred within 6h, followed by disintegration of the damaged myofibres within 12h. Newly formed myofibres appeared by Day 7. 3. The number of MyoD-positive nuclei (activated satellite cells) in the injured muscle was significantly increased by 6h, reaching a maximum by 12h after contusion. However, the number of MyoD-positive nuclei decreased towards control levels by Day 7. Changes in the number of bromodeoxyuridine-labelled nuclei (proliferating satellite cells) paralleled the changes seen in the number of MyoD-positive nuclei. Conversely, expression of myogenin protein was not apparent until Day 3 and increased further by Day 7. Colabelling of MyoD and myogenin was seen in only a few cells. 4. The time-course of MyoD mRNA expression corresponded with MyoD protein expression. However, there were two peaks in myogenin mRNA expression: 6h and Day 7 after contusion. The second peak coincided with upregulation of myostatin mRNA levels. 5. The results of the present study suggest that contusion activates a homogeneous population of satellite cells to proliferate within 3days, followed by differentiation to form new myofibres. The latter may be regulated, in part, by myostatin.

Oncostatin M inhibits myoblast differentiation and regulates muscle regeneration

Oncostatin M (OSM) is a cytokine of the interleukin-6 family and plays important roles during inflammation. However, its roles in myoblast differentiation and muscle regeneration remain unexplored. We show here that OSM potently inhibited myoblast differentiation mainly by activating the JAK1/STAT1/STAT3 pathway. OSM downregulated myocyte enhancer-binding factor 2A (MEF2A), upregulated the expression of Id1 and Id2, and inhibited the transcriptional activity of MyoD and MEF2. In addition, OSM also enhanced the expression of STAT3 and OSM receptor, which constituted a positive feedback loop to further amplify OSM-induced signaling. Moreover, we found that STAT1 physically associated with MEF2 and repressed its transcriptional activity, which could account for the OSM-mediated repression of MEF2. Although undetectable in normal muscles in vivo, OSM was rapidly induced on muscle injury and then promptly downregulated just before the majority of myoblasts differentiate. Prolonged expression of OSM in muscles compromised the regeneration process without affecting myoblast proliferation, suggesting that OSM functions to prevent proliferating myoblasts from premature differentiation during the early phase of muscle regeneration.

The expression pattern of myogenic regulatory factors MyoD, Myf6 and Pax7 in postnatal porcine skeletal muscles

The MyoD, Myf6 genes, which belong to the family of muscle regulatory factors (MRFs) play a major role in muscle growth and development. Therefore, they are considered as candidate genes for meat production traits in pigs. These basic helix-loop-helix (bHLH) transcription factors regulate myogenesis: they initiate the formation of muscle fibres and regulate the transcription of muscle specific genes. The paired-box transcription factor Pax7 plays critical roles during fetal development and this protein is essential for renewal and maintenance of muscle stem cells. In particular, expression of Pax7 and MyoD is correlated with presence of active satellite cells, important in hyperplastic and hypertrophic growth in skeletal muscle. The objective of the study was to investigate the level of expression of MyoD, Myf6 and Pax7 genes in porcine skeletal muscles (m. semimembranosus, m. biceps femoris, m. gracilis) in breeds differing in muscularity. Moreover, we investigated expression profile of these genes during ontogenesis in Polish Large White (PLW) and Pietrain pigs in the largest ham muscle (m.semimembranosus). Analysis of several ham muscles showed higher expression of MyoD in the Polish Landrace (PL) breed than in Pietrain and PLW pigs (m. semimembranosus P<0.001; m. biceps femoris P<0.05 and P<0.01, respectively; m. gracilisP<0.01). The level of Pax7 transcript depended on type of muscle and breed. The highest expression was in m. gracilis in Pietrain and the lowest in Polish Landrace. Our results indicate that MyoD and Pax7 genes had higher expression levels in the early stages of development in both investigated breeds. The total expression profile of MyoD and Pax7 genes suggests that higher muscularity in Pietrain pigs is associated with the presence of a greater number of active satellite stem cells compared to other breeds. The expression level of Myf6 gene does not indicate significant differences between muscles, ages and breeds.

Heterogeneity in the muscle satellite cell population

Satellite cells, the adult stem cells responsible for skeletal muscle regeneration, are defined by their location between the basal lamina and the fiber sarcolemma. Increasing evidence suggests that satellite cells represent a heterogeneous population of cells with distinct embryological origin and multiple levels of biochemical and functional diversity. This review focuses on the rich diversity of the satellite cell population based on studies across species. Ultimately, a more complete characterization of the heterogeneity of satellite cells will be essential to understand the functional significance in terms of muscle growth, homeostasis, tissue repair, and aging.

Regulatory factors and cell populations involved in skeletal muscle regeneration

Skeletal muscle regeneration is a complex process, which is not yet completely understood. Satellite cells, the skeletal muscle stem cells, become activated after trauma, proliferate, and migrate to the site of injury. Depending on the severity of the myotrauma, activated satellite cells form new multinucleated myofibers or fuse to damaged myofibers. The specific microenvironment of the satellite cells, the niche, controls their behavior. The niche contains several components that maintain satellite cells quiescence until they are activated. In addition, a great diversity of stimulatory and inhibitory growth factors such as IGF-1 and TGF-beta1 regulate their activity. Donor-derived satellite cells are able to improve muscle regeneration, but their migration through the muscle tissue and across endothelial layers is limited. Less than 1% of their progeny, the myoblasts, survive the first days upon intra-muscular injection. However, a range of other multipotent muscle- and non-muscle-derived stem cells are involved in skeletal muscle regeneration. These stem cells can occupy the satellite cell niche and show great potential for the treatment of skeletal muscle injuries and diseases. The aim of this review is to discuss the niche factors, growth factors, and other stem cells, which are involved in skeletal muscle regeneration. Knowledge about the factors regulating satellite cell activity and skeletal muscle regeneration can be used to improve the treatment of muscle injuries and diseases.

TAZ as a novel enhancer of MyoD-mediated myogenic differentiation

Myoblast differentiation is indispensable for skeletal muscle formation and is governed by the precisely coordinated regulation of a series of transcription factors, including MyoD and myogenin, and transcriptional coregulators. TAZ (transcriptional coactivator with PDZ-binding motif) has been characterized as a modulator of mesenchymal stem cell differentiation into osteoblasts and adipocytes through its regulation of lineage-specific master transcription factors. In this study, we investigated whether TAZ affects myoblast differentiation, which is one of the differentiated lineages of mesenchymal stem cells. Ectopic overexpression of TAZ in myoblasts increases myogenic gene expression in a MyoD-dependent manner and hastens myofiber formation, whereas TAZ knockdown delays myogenic differentiation. In addition, enforced coexpression of TAZ and MyoD in fibroblasts accelerates MyoD-induced myogenic differentiation. TAZ physically interacts with MyoD through the WW domain and activates MyoD-dependent gene transcription. TAZ additionally enhances the interaction of MyoD with the myogenin gene promoter. These results strongly suggest that TAZ functions as a novel transcriptional modulator of myogenic differentiation by promoting MyoD-mediated myogenic gene expression.

MyoD expression profile and developmental differences of leg and breast muscle in Peking duck (Anas platyrhynchos Domestica) during embryonic to neonatal stages

In order to investigate the developmental differences between the duck breast muscle and leg muscle tissues during the embryonic stage to neonatal stages, as well as the expression profile of MyoD between the two muscle tissues, the morphologic characteristics in the two muscle tissues during duck embryo stages at E14, E18, E22, E27 and D7 were compared through the muscle paraffin sections. The coding domain sequence of duck MyoD gene was cloned, and then the expression of MyoD in duck leg muscle and breast muscle during embryo stage on E10, E14, E18, E22, E27 and D7 was detected using qRT-PCR method. Results showed that the developmental status of the duck breast muscle in embryonic phrases lag behind that of leg muscle. The CDS of duck MyoD gene consists of 894 nucleotides, and showed relatively high similarity with the gene of other species. The MyoD mRNA expressed in both kinds of muscle tissues and the expression profile had a similar trend, although the expression level of MyoD in the breast muscle was significantly higher than that in the leg muscle at each developmental stages (p<0.05). Results suggested that MyoD might have potential functions in controlling muscle fiber phenotype during the secondary myogenesis of muscle development. These fundamental works may provide some valuable clues for knowing the roles of MyoD in the myogenesis and the muscle fiber type differentiation in birds.

Down-regulation of MyoD by calpain 3 promotes generation of reserve cells in C2C12 myoblasts

Calpain 3 is a calcium-dependent cysteine protease that is primarily expressed in skeletal muscle and is implicated in limb girdle muscular dystrophy type 2A. To date, its best characterized function is located within the sarcomere, but this protease is found in other cellular compartments, which suggests that it exerts multiple roles. Here, we present evidence that calpain 3 is involved in the myogenic differentiation process. In the course of in vitro culture of myoblasts to fully differentiated myotubes, a population of quiescent undifferentiated "reserve cells" are maintained. These reserve cells are closely related to satellite cells responsible for adult muscle regeneration. In the present work, we observe that reserve cells express higher levels of endogenous Capn3 mRNA than proliferating myoblasts. We show that calpain 3 participates in the establishment of the pool of reserve cells by decreasing the transcriptional activity of the key myogenic regulator MyoD via proteolysis independently of the ubiquitin-proteasome degradation pathway. Our results identify calpain 3 as a potential new player in the muscular regeneration process by promoting renewal of the satellite cell compartment.

Clonal characterization of rat muscle satellite cells: proliferation, metabolism and differentiation define an intrinsic heterogeneity

Satellite cells (SCs) represent a distinct lineage of myogenic progenitors responsible for the postnatal growth, repair and maintenance of skeletal muscle. Distinguished on the basis of their unique position in mature skeletal muscle, SCs were considered unipotent stem cells with the ability of generating a unique specialized phenotype. Subsequently, it was demonstrated in mice that opposite differentiation towards osteogenic and adipogenic pathways was also possible. Even though the pool of SCs is accepted as the major, and possibly the only, source of myonuclei in postnatal muscle, it is likely that SCs are not all multipotent stem cells and evidences for diversities within the myogenic compartment have been described both in vitro and in vivo. Here, by isolating single fibers from rat flexor digitorum brevis (FDB) muscle we were able to identify and clonally characterize two main subpopulations of SCs: the low proliferative clones (LPC) present in major proportion (approximately 75%) and the high proliferative clones (HPC), present instead in minor amount (approximately 25%). LPC spontaneously generate myotubes whilst HPC differentiate into adipocytes even though they may skip the adipogenic program if co-cultured with LPC. LPC and HPC differ also for mitochondrial membrane potential (DeltaPsi(m)), ATP balance and Reactive Oxygen Species (ROS) generation underlying diversities in metabolism that precede differentiation. Notably, SCs heterogeneity is retained in vivo. SCs may therefore be comprised of two distinct, though not irreversibly committed, populations of cells distinguishable for prominent differences in basal biological features such as proliferation, metabolism and differentiation. By these means, novel insights on SCs heterogeneity are provided and evidences for biological readouts potentially relevant for diagnostic purposes described.

Nandrolone stimulates MyoD expression during muscle regeneration in the condition of myonecrosis induced by Bothrops jararacussu venom poisoning

Myonecrosis with permanent loss of muscle mass is a relevant local toxic effect following envenomation with Bothrops jararacussu snake venom. Regeneration of adult skeletal muscle involves the activation of satellite cells, a process regulated by myogenic regulatory factors (MRF). MyoD is an MRF involved in both proliferation and differentiation of satellite cells. Androgens are modulators of skeletal muscle, known to increase muscle mass and strength. This study examined the hypothesis that anabolic androgens improve the muscle regeneration process in mice following envenomation by Bothrops jararacussu snake venom. Myonecrosis was induced by venom injection (30 microg/50 microl in physiological solution) over the extensor digitorum longus (EDL) muscles of mice. Nandrolone (ND) (6 mg/kg, sc) was administered after 12 h, 7 d, and 14 d following venom injection. The histological changes in EDL muscle at 1, 3, 7, and 21 d after muscle injury were analyzed by light microscopy. Cross-sectional areas of fibers were measured. MyoD was evaluated by immunofluorescence technique. Histological examination revealed the presence of a regeneration process in ND-treated animals, characterized by the appearance of some myotubes at 3 d, and numerous myotubes at 7 d from venom injection. Nandrolone treatment reduced the frequency of small fibers at 7 and 21 d after venom administration, and increased the frequency of large fibers at 7 d postinjury. Nandrolone also significantly augmented the expression of MyoD-positive cells at 7 and 21 d after envenomation. These results suggest that ND accelerates muscle regeneration and indicate the involvement of MyoD in this process.

Heat stress increases myonuclear number and fiber size via satellite cell activation in rat regenerating soleus fibers

To investigate the effects of heat stress (hyperthermia) on muscle degeneration-regeneration, the soleus muscles of adult male Wistar rats were injected bilaterally with a single injection of bupivacaine. The rats were assigned to a sedentary control (Con), heat stress (Heat), bupivacaine-injected (BPVC), or bupivacaine-injected plus heat stress (BPVC+Heat) group. Heat stress was induced in the Heat and BPVC+Heat groups by immersion of the lower half of the body into water maintained at 42 ± 1°C for 30 min 48 h after the injection of bupivacaine and every other day during the following 1 or 2 wk. The soleus muscles in all groups were excised 24 h after the final bout of heat stress. Mean muscle weight, fiber cross-sectional area, myonuclear number, and heat shock protein 72 (Hsp72) and calcineurin protein levels were lower in the BPVC than in the Con or Heat groups at both time points. In contrast, several of these parameters in the BPVC+Heat group were not different or higher than in the Con or Heat groups at the 1- and/or 2-wk time points. The number of total and activated satellite cells, estimated by analyses of Pax7-negative, M-cadherin-negative, and MyoD-positive nuclei, was greater in BPVC+Heat than in all other groups. Combined, the results indicate that heat stress-related activation of satellite cells and upregulation of Hsp72 and calcineurin expression played important roles in the regeneration of the soleus fibers after bupivacaine injection.

The origin of intermuscular adipose tissue and its pathophysiological implications

The intermuscular adipose tissue (IMAT) is a depot of adipocytes located between muscle bundles. Several investigations have recently been carried out to define the phenotype, the functional characteristics, and the origin of the adipocytes present in this depot. Among the different mechanisms that could be responsible for the accumulation of fat in this site, the dysdifferentiation of muscle-derived stem cells or other mesenchymal progenitors has been postulated, turning them into cells with an adipocyte phenotype. In particular, muscle satellite cells (SCs), a heterogeneous stem cell population characterized by plasticity and self-renewal that allow muscular growth and regeneration, can acquire features of adipocytes, including the abilities to express adipocyte-specific genes and accumulate lipids. Failure to express the transcription factors that direct mesenchymal precursors into fully differentiated functionally specialized cells may be responsible for their phenotypic switch into the adipogenic lineage. We proved that human SCs also possess a clear adipogenic potential that could explain the presence of mature adipocytes within skeletal muscle. This occurs under some pathological conditions (i.e., primary myodystrophies, obesity, hyperglycemia, high plasma free fatty acids, hypoxia, etc.) or as a consequence of thiazolidinedione treatment or simply because of a sedentary lifestyle or during aging. Several pathways and factors (PPARs, WNT growth factors, myokines, GEF-GAP-Rho, p66(shc), mitochondrial ROS production, PKCβ) could be implicated in the adipogenic conversion of SCs. The understanding of the molecular pathways that regulate muscle-to-fat conversion and SC behavior could explain the increase in IMAT depots that characterize many metabolic diseases and age-related sarcopenia.

Skeletal muscle stem cells in developmental versus regenerative myogenesis

Tissue and organ regeneration proceed in a coordinated manner to restore proper function after trauma. Vertebrate skeletal muscle has a remarkable ability to regenerate after repeated and complete destruction of the tissue, yet limited information is available on how muscle stem and progenitor cells, and other nonmuscle cells, reestablish homeostasis after the regenerative process. The genetic pathways that regulate the establishment of skeletal muscle in the embryo have been studied extensively, and many of the genes that govern muscle stem cell maintenance and commitment are redeployed during adult homeostasis and regeneration. Therefore, correlates can be made between embryonic muscle development and postnatal regeneration. However, there are some important distinctions between prenatal development and regeneration - in the context of the cells, niche, anatomy and the regulatory genes employed. The similarities and distinctions between these two scenarios are the focus of this review.

Alterations in mRNA and protein levels of metalloproteinases-2, -9, and -14 and tissue inhibitor of metalloproteinase-2 responses to traumatic skeletal muscle injury

This study characterizes the temporal relationship of membrane type-1 matrix metalloproteinase (MT1-MMP) and tissue inhibitor of metalloproteinase-2 (TIMP-2) expression in skeletal muscle following injury. Tibialis anterior (TA) muscles from 60 mice were exposed and injured by applying a cold steel probe (-79 degrees C) to the muscle for 10 s. Thereafter, TA muscles from uninjured and injured legs were collected at 3, 10, 24, 48, and 72 h postinjury for analysis of local MT1-MMP, TIMP-2, and matrix metalloproteinases-2 and -9 (MMP-2 and MMP-9) mRNA and protein content via quantitative RT-PCR, immunoblotting, zymography, and immunofluorescence. All data are expressed as fold change of injured leg vs. uninjured leg. MT1-MMP mRNA levels were decreased significantly at 48 and 72 h postinjury by approximately 9- and 21-fold, respectively (P < 0.01). Both TIMP-2 and MMP-2 mRNA expression significantly decreased in the injured leg by approximately 4- to 10-fold at 10-72 h postinjury (P < 0.01). MMP-9 mRNA expression was significantly increased at 10, 24, and 48 h postinjury by 6- (P < 0.05), 25-, and 12-fold (P < 0.01), respectively. Protein content of latent (63 kDa) MT1-MMP was decreased at 48 and 72 h postinjury by approximately 2-fold (P < 0.01). Content of the soluble (50 kDa) fragment of MT1-MMP was significantly increased by approximately 17-, 25-, and 67-fold at 24 (P < 0.05), 48, and 72 h (P < 0.01) postinjury, respectively. TIMP-2 protein levels diminished from 3 to 48 h postinjury by 1.5-fold to 1.8-fold (P < 0.01), before returning to baseline levels at 72 h postinjury. Zymography revealed visual increases in gelatinase activity in molecular weight regions corresponding to MMP-9 and MMP-2. In conclusion, skeletal muscle injury initiates a sequence of events in the MT1-MMP proteolytic cascade resulting in elevated levels of the soluble (50 kDa) fragment of MT1-MMP, which could enhance pericellular extracellular matrix remodeling.

Protective effects of insulin-like growth factor-I on the decrease in myogenic differentiation by ionizing radiation

PURPOSE

The aim of the work is to evaluate the effects of insulin-like growth factor-1 (IGF-1) on the decrease in myotube formation induced by ionizing radiation.

MATERIALS AND METHODS

We induced C2C12 cells to a myogenic linage following X-ray irradiation at 2 and 4 Gy. Myogenic differentiation was estimated using immnocytochemical staining with anti-myosin antibody, and the anti-myosin antibody positive areas, the total number of nuclei, the number of nuclei included in multinucleated myotubes per field, and the myotube formation ratio were analyzed.

RESULTS

In the myogenic differentiation in the presence of IGF-1, the decrease in anti-myosin antibody positive areas, the nuclei included in myotubes, and the myotube formation ratio induced by X-ray irradiation at 2 Gy was restored to control levels.

CONCLUSIONS

The addition of IGF-1 protected against the decrease myotube formation induced by X-ray irradiation at 2 Gy. Since X-ray irradiation at 2 Gy is usually used for multi-fractionated irradiation in radiotherapy, our findings suggest that IGF-1 could be useful to protect against impairment of muscle repair induced by therapeutic dose radiation.

Mechanisms of lipase maturation

Lipases are acyl hydrolases that represent a diverse group of enzymes present in organisms ranging from prokaryotes to humans. This article focuses on an evolutionarily related family of extracellular lipases that include lipoprotein lipase, hepatic lipase and endothelial lipase. As newly synthesized proteins, these lipases undergo a series of co- and post-translational maturation steps occurring in the endoplasmic reticulum, including glycosylation and glycan processing, and protein folding and subunit assembly. This article identifies and discusses mechanisms that direct early and late events in lipase folding and assembly. Lipase maturation employs the two general chaperone systems operating in the endoplasmic reticulum, as well as a recently identified lipase-specific chaperone termed lipase maturation factor 1. We propose that the two general chaperone systems act in a coordinated manner early in lipase maturation in order to help create partially folded monomers; lipase maturation factor 1 then facilitates final monomer folding and subunit assembly into fully functional homodimers. Once maturation is complete, the lipases exit the endoplasmic reticulum and are secreted to extracellular sites, where they carry out a number of functions related to lipoprotein and lipid metabolism.

Niche regulation of muscle satellite cell self-renewal and differentiation

Muscle satellite cells have been shown to be a heterogeneous population of committed myogenic progenitors and noncommitted stem cells. This hierarchical composition of differentiating progenitors and self-renewable stem cells assures the extraordinary regenerative capacity of skeletal muscles. Recent studies have revealed a role for asymmetric division in satellite cell maintenance and offer novel insights into the regulation of satellite cell function by the niche. A thorough understanding of the molecular regulation and cell fate determination of satellite cells and other potential stem cells resident in muscle is essential for successful stem cell-based therapies to treat muscular diseases.

Dietary carbohydrate level affects transcription factor expression that regulates skeletal muscle myogenesis in rainbow trout

Understanding the effects of dietary carbohydrates on transcription factors that regulate myogenesis provides insight into the role of nutrient sensing by satellite cells towards myocyte differentiation. We evaluated the influence of dietary carbohydrate level (0, 15, 25 or 35%) on the temporal mRNA expression patterns (4, 8 or 12 weeks) of transcription factors that regulate satellite cell myocyte addition (MA) in rainbow trout (Oncorhynchus mykiss), a vertebrate with indeterminate growth. Relative to the 0% carbohydrate (NC) diet, 15 (IC-15) and 25% (IC-25) carbohydrate containing diets significantly up-regulate MyoD and Myf5, but not Pax7, after 12 weeks of feeding. Simultaneously, the Pax7/MyoD mRNA expression ratio declined significantly with both the IC diets. Myogenin mRNA expression also increased in rainbow trout (RBT) fed the IC-15 diet. The high carbohydrate (HC) diet (35%) attenuated the increased mRNA expression of these transcription factors. It is of note that the 4 and 8 week samples lacked the promyogenic expression patterns. The myogenic gene expression in fish fed the IC-15 diet for 12 weeks indicate a transcriptional signature that reflects increased satellite cell myogenesis. Our results suggest a potential role for satellite cells in the nutrient sensing ability of a vertebrate with indeterminate skeletal muscle growth.

Role of MyoD in denervated, disused, and exercised muscle

The myogenic regulatory factor MyoD plays an important role in embryonic and adult skeletal muscle growth. Even though it is best known as a marker for activated satellite cells, it is also expressed in myonuclei, and its expression can be induced by a variety of different conditions. Several model systems have been used to study the mechanisms behind MyoD regulation, such as exercise, stretch, disuse, and denervation. Since MyoD reacts in a highly muscle-specific manner, and its expression varies over time and between species, universally valid predictions and explanations for changes in MyoD expression are not possible. This review explores the complex role of MyoD in muscle plasticity by evaluating the induction of MyoD expression in the context of muscle composition and electrical and mechanical stimulation.

The ubiquitin-proteasome and the mitochondria-associated apoptotic pathways are sequentially downregulated during recovery after immobilization-induced muscle atrophy

Immobilization produces morphological, physiological, and biochemical alterations in skeletal muscle leading to muscle atrophy and long periods of recovery. Muscle atrophy during disuse results from an imbalance between protein synthesis and proteolysis but also between apoptosis and regeneration processes. This work aimed to characterize the mechanisms underlying muscle atrophy and recovery following immobilization by studying the regulation of the mitochondria-associated apoptotic and the ubiquitin-proteasome-dependent proteolytic pathways. Animals were subjected to hindlimb immobilization for 4-8 days (I4 to I8) and allowed to recover after cast removal for 10-40 days (R10 to R40). Soleus and gastrocnemius muscles atrophied from I4 to I8 to a greater extent than extensor digitorum longus and tibialis anterior muscles. Gastrocnemius muscle atrophy was first stabilized at R10 before being progressively reduced until R40. Polyubiquitinated proteins accumulated from I4, whereas the increased ubiquitination rates and chymotrypsin-like activity of the proteasome were detectable from I6 to I8. Apoptosome and caspase-3 or -9 activities increased at I6 and I8, respectively. The ubiquitin-proteasome-dependent pathway was normalized early when muscle stops to atrophy (R10). By contrast, the mitochondria-associated apoptotic pathway was first downregulated below basal levels when muscle started to recover at R15 and completely normalized at R20. Myf 5 protein levels decreased from I4 to I8 and were normalized at R10. Altogether, our results suggest a two-stage process in which the ubiquitin-proteasome pathway is rapidly up- and downregulated when muscle atrophies and recovers, respectively, whereas apoptotic processes may be involved in the late stages of atrophy and recovery.

Co-expression of IGF-1 family members with myogenic regulatory factors following acute damaging muscle-lengthening contractions in humans

Muscle regeneration following injury is dependent on the ability of muscle satellite cells to activate, proliferate and fuse with damaged fibres. This process is controlled by the myogenic regulatory factors (MRF). Little is known about the temporal relation of the MRF with the expression of known myogenic growth factors (i.e. IGF-1) in humans following muscle damage. Eight subjects (20.6 +/- 2.1 years; 81.4 +/- 9.8 kg) performed 300 lengthening contractions (180 deg s(-1)) of their knee extensors in one leg on a dynamometer. Blood and muscle samples were collected before and at 4 (T4), 24 (T24), 72 (T72) and 120 h (T120) post-exercise. Mechano growth factor (MGF), IGF-1Ea and IGF-1Eb mRNA were quantified. Serum IGF-1 did not change over the post-exercise time course. IGF-1Ea and IGF-1Eb mRNA increased approximately 4- to 6-fold by T72 (P < 0.01) and MGF mRNA expression peaked at T24 (P = 0.005). MyoD mRNA expression increased approximately 2-fold at T4 (P < 0.05). Myf5 expression peaked at T24 (P < 0.05), while MRF4 and myogenin mRNA expression peaked at T72 (P < 0.05). Myf5 expression strongly correlated with the increase in MGF mRNA (r(2) = 0.83; P = 0.03), while MRF4 was correlated with both IGF-1Ea and -Eb (r(2) = 0.90; r(2) = 0.81, respectively; P < 0.05). Immunofluorescence analysis showed IGF-1 protein expression localized to satellite cells at T24, and to satellite cells and the myofibre at T72 and T120; IGF-1 was not detected at T0 or T4. These results suggest that the temporal response of MGF is probably related to the activation/proliferation phase of the myogenic programme as marked by an increase in both Myf5 and MyoD, while IGF-1Ea and -Eb may be temporally related to differentiation as marked by an increase in MRF4 and myogenin expression following acute muscle damage.

PAX3/7 expression coincides with MyoD during chronic skeletal muscle overload

Paired box (Pax) proteins 3 and 7 are key determinants for embryonic skeletal muscle development by initiating myogenic regulatory factor (MRF) gene expression. We show that Pax3 and 7 participate in adult skeletal muscle plasticity during the initial responses to chronic overload (< or =7 days) and appear to coordinate MyoD expression, a member of the MRF family of genes. Pax3 and 7 mRNA were higher than control within 12 h after initiation of overload, preceded the increase in MyoD mRNA on day 1, and peaked on day 2. On days 3 and 7, Pax7 mRNA remained higher than control, suggesting that satellite cell self-renewal was occurring. Pax3 and 7 and MyoD protein levels were higher than control on days 2 and 3. These data indicate that Pax3 and 7 coordinate the recapitulation of developmental-like regulatory mechanisms in response to growth-inducing stimuli in adult skeletal muscle, presumably through activation of satellite cells.

Cdk9-55: a new player in muscle regeneration

Adult skeletal muscle contains a specialized population of myogenic quiescent stem cells, termed satellite cells, which contribute to repair myofibers after injury. During muscle regeneration, satellite cells exit their normal quiescent state, proliferate, activating MyoD and Myf-5 expression, and finally differentiate and fuse to reconstitute the injured muscle architecture. We have previously reported that cdk9 is required for myogenesis in vitro by activating MyoD-dependent transcription. In myoblasts induced to differentiate, MyoD recruits cdk9 on the chromatin of muscle-specific regulatory regions. This event correlates with chromatin-modifying enzyme recruitment and phosphorylation of cdk9-specific target residues at the carboxyl-terminal domain of RNA polymerase II. Here we report that a second cdk9 isoform, termed cdk9-55, plays a fundamental role in muscle regeneration and differentiation in vivo. This alternative form is specifically induced in injured myofibers and its activity is strictly required for the completion of muscle regeneration process.

Trichinella spiralis: nurse cell formation with emphasis on analogy to muscle cell repair

Trichinella infection results in formation of a capsule in infected muscles. The capsule is a residence of the parasite which is composed of the nurse cell and fibrous wall. The process of nurse cell formation is complex and includes infected muscle cell response (de-differentiation, cell cycle re-entry and arrest) and satellite cell responses (activation, proliferation and differentiation). Some events that occur during the nurse cell formation are analogous to those occurring during muscle cell regeneration/repair. This article reviews capsule formation with emphasis on this analogy.

Age-related reductions in expression of serum response factor and myocardin-related transcription factor A in mouse skeletal muscles

The molecular signaling pathways linking the atrophy of skeletal muscle during aging have not been identified. Using reverse transcription (RT)-PCR, Western blotting, and immunofluorescence microscopy, we investigated whether the amounts of RhoA, RhoGDI, SRF, MRTF-A, and MyoD in the triceps brachii and quadriceps muscles change with aging in mice. Young adult (3 mo) and aged (24 mo) C57BL/6J mice were used. Senescent mice possessed many fibers with central nuclei in the quadriceps muscle. Western blotting using a homogenate of whole muscle or the cytosolic fraction clearly showed that the amount of SRF protein was significantly decreased in the aged skeletal muscles. Immunofluorescence labeling indicated more SRF-positive muscle fibers in young mice. Both young and old mice possessed SRF immunoreactivity in some satellite cells expressing Pax7. MRTF-A and STARS mRNA levels significantly declined with aging in the triceps brachii and quadriceps muscles. The amount of MRTF-A protein was markedly reduced in the nuclear fraction of aged muscle of mice. The amounts of RhoA and RhoGDI in the crude homogenate or the cytosolic and membrane fractions were greater in the aged muscle. Senescent mice possessed significantly higher levels of MyoD protein in the cytosol and nucleus. Decreased SRF and MRTF expression may induce the atrophy of skeletal muscle with aging.

Glucocorticoids differentially regulate degradation of MyoD and Id1 by N-terminal ubiquitination to promote muscle protein catabolism

Accelerated protein degradation via the ubiquitin-proteasome pathway is the principal cause of skeletal muscle wasting associated with common human disease states and pharmacological treatment with glucocorticoids. Although many protein regulatory factors essential for muscle development and regeneration are degraded via the ubiquitin system, little is known about the mechanisms and regulation of this pathway that promote wasting muscle. Here, we demonstrate that, in differentiated myotubes, glucocorticoid, via the glucocorticoid receptor, selectively induces a decrease in protein abundance of MyoD, a master switch for muscle development and regeneration, but not that of its negative regulator Id1. This decrease in MyoD protein results from accelerated degradation after glucocorticoid exposure. Using MyoD and Id1 mutants deficient in either N terminus-dependent or internal lysine-dependent ubiquitination, we further show that these ubiquitination pathways of MyoD degradation are regulated differently from those of Id1 degradation. Specifically, glucocorticoid activates the N-terminal ubiquitination pathway in MyoD degradation in myotubes, without concomitant effects on Id1 degradation. This effect of glucocorticoid on MyoD and Id1 protein degradation is associated with the distinct cellular compartments in which their degradation occurs. Taken together, these results support a key role for the N terminus-dependent ubiquitination pathway in the physiology of muscle protein degradation.

Osterix overexpression enhances osteoblast differentiation of muscle satellite cells in vitro

Muscle satellite cells have long been considered a distinct myogenic lineage responsible for postnatal growth, repair and maintenance of skeletal muscle. Recent studies have demonstrated that they are multi-potential. Osterix (Osx), a novel zinc-finger-containing transcription factor of the sp family, is required for osteoblast differentiation and bone formation. It was hypothesized that Osx overexpression would enhance osteoblast differentiation of muscle satellite cells in vitro. Recombinant adenovirus-mediated Osx gene (Ad-Osx) was constructed and used to transfect muscle satellite cells. Osx overexpression inhibited myogenesis, as demonstrated by suppression of MyoD and myogenin mRNA levels and reduced myotube formation. Muscle satellite cells transduced with Ad-Osx exhibited apparent osteoblast differentiation as determined by the expression of related osteoblastic genes, increased activity of alkaline phosphatase and the formation of mineralized nodules. These results confirmed the ability of Osx to enhance osteoblast differentiation of muscle satellite cells in vitro, and the competence of muscle satellite cells as promising seed cells for bone tissue engineering.

Structural changes in the lengthened rabbit muscle

This study evaluated the histological changes in muscle tissue after limb lengthening in skeletally mature and immature rabbits and assessed the most vulnerable level of striated muscle. Twenty-three male domestic white rabbits, divided into six groups, were operated on and different lengthening protocols were used in the mature and immature rabbits. The histopathological changes were analysed by a semi-quantitative method according to the scoring system of Lee et al. (Acta Orthop Scand 64(6):688-692, 1993). After the evaluation of the five main degenerative parameters (muscle atrophy, muscle nuclei internalisation, degeneration of the muscle fibre, perimysial and endomysial fibrosis, haematomas), it is evident that the adults lengthened at a rate of 1.6 mm/day showed more degenerative changes than those lengthened at 0.8 mm/day. The adult 1.6 mm/day lengthened group presented significantly higher damage in the muscle and lower regenerative signs compared with the young 1.6 mm/day lengthened group, according to the summarised degenerative scores.

A naturally occurring calcineurin variant inhibits FoxO activity and enhances skeletal muscle regeneration

The calcium-activated phosphatase calcineurin (Cn) transduces physiological signals through intracellular pathways to influence the expression of specific genes. Here, we characterize a naturally occurring splicing variant of the CnAβ catalytic subunit (CnAβ1) in which the autoinhibitory domain that controls enzyme activation is replaced with a unique C-terminal region. The CnAβ1 enzyme is constitutively active and dephosphorylates its NFAT target in a cyclosporine-resistant manner. CnAβ1 is highly expressed in proliferating myoblasts and regenerating skeletal muscle fibers. In myoblasts, CnAβ1 knockdown activates FoxO-regulated genes, reduces proliferation, and induces myoblast differentiation. Conversely, CnAβ1 overexpression inhibits FoxO and prevents myotube atrophy. Supplemental CnAβ1 transgene expression in skeletal muscle leads to enhanced regeneration, reduced scar formation, and accelerated resolution of inflammation. This unique mode of action distinguishes the CnAβ1 isoform as a candidate for interventional strategies in muscle wasting treatment.